libxl_json: Introduce libxl__json_object_to_yajl_gen.

libxl_json: Introduce libxl__json_object_to_yajl_gen.

This function converts a libxl__json_object to yajl by calling every
yajl_gen_* function on a preallocated yajl_gen hand.

This helps to integrate a json_object into an already existing
yajl_gen tree.

This function is used in a later patch.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- renamed local variable "index" to "idx" to avoid clash with
         index(3) function, highlighted by Wshadow ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>